fix: prevent issue label workflow from triggering on pull request events

Author: JhaSourav07

.github/workflows/milestone-manager.yml

@@ -13,7 +13,7 @@ permissions:
 jobs:
   assign-to-milestone:
     if: >
-      (github.event_name == 'issues' && github.event.label.name == 'GSSoC 2026') ||
+      (github.event_name == 'issues' && github.event.label.name == 'GSSoC 2026' && github.event.issue.pull_request == null) ||
       (github.event_name == 'pull_request_target' && github.event.label.name == 'gssoc:approved')
     runs-on: ubuntu-latest
     steps: